1. Field of the Invention
This invention generally relates to a fabrication method of a semiconductor device, and more particularly, to a method of fabricating a GaN-based semiconductor device.
2. Description of the Related Art
Semiconductor devices that employ a nitride-based compound semiconductor, in particular, the semiconductor devices that employ gallium nitride (GaN) (hereinafter, referred to as GaN-based semiconductor devices) are used as power devices that operate at high frequencies and high power. Field effect transistors are especially known as semiconductor devices that are suitable for amplification in high-frequency bands such as microwave, millimeter wave, and sub-millimeter wave. In a field effect transistor, a source electrode, drain electrode, and gate electrode are formed on a GaN semiconductor substrate on which layers such as an AlGaN supply layer, GaN cap layer, and the like are deposited.
The field effect transistor having the aforementioned structure can be fabricated in a production process such as organic processing, oxygen ashing, dry etching, and the like. The organic processing denotes a process that uses a developer for forming the pattern. The oxygen ashing denotes a process that removes the organic remainder after the pattern is formed.
In the process such as organic processing, oxygen ashing, dry etching, and the like, an affected layer is generated on a surface of the GaN cap layer. As an analysis result of the affected layer generated on the surface of the GaN cap layer by use of EDX, it has been confirmed that the affected layer includes GaO. A path of leakage current that flows between the gate and drain, which is thought to be caused by the affected layer, lowers the breakdown voltage.